Ping [PATCH V2] libcpp: Optimize #pragma once with a hash table [PR58770]

[David Malcolm]

On Fri, 2022-08-19 at 13:27 -0700, Paul Hollinsky wrote:
> Hi all,
> 
> Would love some feedback on this patch!
> 
> Thanks,
> Paul

Hi Paul. Sorry for not getting back to you before.  I'm listed as a
libcpp maintainer, but this happens to be a part of libcpp I've not
looked at (I'm mostly just familiar with location_t management). 

FWIW, the patch seems sane to me, though I have one concern: does it
work with precompiled headers?  (given that PCH is implemented by
taking a snapshot of the gc heap and reconstructing it on load, it thus
tends to be an annoying source of bugs when trying the kind of cleanup
this patch is doing).

Sorry to not look be able to look at things in more detail (I'm
travelling)

Hope this is constructive
Dave

> 
> On Mon, Aug 01, 2022 at 05:18:40AM +0000, Paul Hollinsky wrote:
> > Rather than traversing the all_files linked list for every include,
> > this factors out the quick idempotency checks (modification time
> > and size) to be the keys in a hash table so we can find matching
> > files quickly.
> > 
> > The hash table value type is a linked list, in case more than one
> > file matches the quick check.
> > 
> > The table is only built if a once-only file is seen, so include
> > guard performance is not affected.
> > 
> > My laptop would previously complete Ricardo's benchmark from the
> > PR in ~1.1s using #pragma once, and ~0.35s using include guards.
> > 
> > After this change, both benchmarks now complete in ~0.35s. I did
> > have to randomize the modification dates on the benchmark headers
> > so the files did not all end up in the same hash table list, but
> > that would likely not come up outside of the contrived benchmark.
> > 
> > I bootstrapped and ran the testsuite on x86_64 Darwin, as well as
> > ppc64le and aarch64 Linux.
> > 
> > libcpp/ChangeLog:
> > 
> >         PR preprocessor/58770
> >         * internal.h: Add hash table for #pragma once
> >         * files.cc: Optimize #pragma once with the hash table
> > 
> > Signed-off-by: Paul Hollinsky <paulhollinsky@gmail.com>
> > ---
> >  libcpp/files.cc   | 116
> > +++++++++++++++++++++++++++++++++++++++++++---
> >  libcpp/internal.h |   3 ++
> >  2 files changed, 112 insertions(+), 7 deletions(-)
> > 
> > diff --git a/libcpp/files.cc b/libcpp/files.cc
> > index 24208f7b0f8..d4ffd77578e 100644
> > --- a/libcpp/files.cc
> > +++ b/libcpp/files.cc
> > @@ -167,6 +167,33 @@ struct file_hash_entry_pool
> >    struct cpp_file_hash_entry pool[FILE_HASH_POOL_SIZE];
> >  };
> >  
> > +/* A set of attributes designed to quickly identify obviously
> > different files
> > +   in a hashtable.  Just in case there are collisions, we still
> > maintain a
> > +   list.  These sub-lists can then be checked for #pragma once
> > rather than
> > +   interating through all_files.  */
> > +struct file_quick_idempotency_attrs
> > +{
> > +  file_quick_idempotency_attrs(const _cpp_file *f)
> > +    : mtime(f->st.st_mtime), size(f->st.st_size) {}
> > +
> > +  time_t mtime;
> > +  off_t size;
> > +
> > +  static hashval_t hash (/* _cpp_file* */ const void *p);
> > +};
> > +
> > +/* Sub-list of very similar files kept in a hashtable to check for
> > #pragma
> > +   once.  */
> > +struct file_sublist
> > +{
> > +  _cpp_file *f;
> > +  file_sublist *next;
> > +
> > +  static int eq (/* _cpp_file* */ const void *p,
> > +                /* file_sublist* */ const void *q);
> > +  static void del (/* file_sublist* */ void *p);
> > +};
> > +
> >  static bool open_file (_cpp_file *file);
> >  static bool pch_open_file (cpp_reader *pfile, _cpp_file *file,
> >                            bool *invalid_pch);
> > @@ -849,17 +876,17 @@ has_unique_contents (cpp_reader *pfile,
> > _cpp_file *file, bool import,
> >    if (!pfile->seen_once_only)
> >      return true;
> >  
> > -  /* We may have read the file under a different name.  Look
> > -     for likely candidates and compare file contents to be sure. 
> > */
> > -  for (_cpp_file *f = pfile->all_files; f; f = f->next_file)
> > +  /* We may have read the file under a different name.  We've kept
> > +     similar looking files in this lists under this hash table, so
> > +     check those more thoroughly.  */
> > +  void* ent = htab_find(pfile->pragma_once_files, file);
> > +  for (file_sublist *e = static_cast<file_sublist *> (ent); e; e =
> > e->next)
> >      {
> > +      _cpp_file *f = e->f;
> >        if (f == file)
> >         continue; /* It'sa me!  */
> >  
> > -      if ((import || f->once_only)
> > -         && f->err_no == 0
> > -         && f->st.st_mtime == file->st.st_mtime
> > -         && f->st.st_size == file->st.st_size)
> > +      if ((import || f->once_only) && f->err_no == 0)
> >         {
> >           _cpp_file *ref_file;
> >  
> > @@ -895,6 +922,38 @@ has_unique_contents (cpp_reader *pfile,
> > _cpp_file *file, bool import,
> >    return true;
> >  }
> >  
> > +/* Add the given file to the #pragma once table so it can be
> > +   quickly identified and excluded the next time it's seen.  */
> > +static void
> > +update_pragma_once_table (cpp_reader *pfile, _cpp_file *file)
> > +{
> > +  void **slot = htab_find_slot (pfile->pragma_once_files, file,
> > INSERT);
> > +  if (slot)
> > +    {
> > +      if (!*slot)
> > +       *slot = xcalloc (1, sizeof (file_sublist));
> > +
> > +      file_sublist *e = static_cast<file_sublist *> (*slot);
> > +      while (e->f)
> > +       {
> > +         if (!e->next)
> > +           {
> > +             void *new_sublist = xcalloc(1, sizeof
> > (file_sublist));
> > +             e->next = static_cast<file_sublist *> (new_sublist);
> > +           }
> > +         e = e->next;
> > +       }
> > +
> > +      e->f = file;
> > +    }
> > +  else
> > +    {
> > +      cpp_error (pfile, CPP_DL_ERROR,
> > +                "Unable to create #pragma once table space for
> > %s",
> > +                _cpp_get_file_name (file));
> > +    }
> > +}
> > +
> >  /* Place the file referenced by FILE into a new buffer on the
> > buffer
> >     stack if possible.  Returns true if a buffer is stacked.  Use
> > LOC
> >     for any diagnostics.  */
> > @@ -950,6 +1009,9 @@ _cpp_stack_file (cpp_reader *pfile, _cpp_file
> > *file, include_type type,
> >        if (!has_unique_contents (pfile, file, type == IT_IMPORT,
> > loc))
> >         return false;
> >  
> > +      if (pfile->seen_once_only && file->once_only)
> > +       update_pragma_once_table (pfile, file);
> > +
> >        if (pfile->buffer && file->dir)
> >         sysp = MAX (pfile->buffer->sysp, file->dir->sysp);
> >  
> > @@ -1434,6 +1496,41 @@ nonexistent_file_hash_eq (const void *p,
> > const void *q)
> >    return filename_cmp ((const char *) p, (const char *) q) == 0;
> >  }
> >  
> > +/* Hasher for the #pragma once hash table.  */
> > +hashval_t
> > +file_quick_idempotency_attrs::hash (const void *p)
> > +{
> > +  const _cpp_file *f = static_cast<const _cpp_file *> (p);
> > +  file_quick_idempotency_attrs kh (f);
> > +  return iterative_hash_object (kh, 0);
> > +}
> > +
> > +/* Equality checker for the #pragma once hash table.  */
> > +int
> > +file_sublist::eq (const void *p, const void *q)
> > +{
> > +  /* Just check if the file q would be in the list p. Every
> > +     file in the list should have these attributes the same,
> > +     so we don't need to traverse.  */
> > +  const file_sublist *e = static_cast<const file_sublist *> (p);
> > +  const _cpp_file *f = static_cast<const _cpp_file *> (q);
> > +  return f->st.st_mtime == e->f->st.st_mtime
> > +        && f->st.st_size == e->f->st.st_size;
> > +}
> > +
> > +/* Cleanup for a file sub-list. Does not free the _cpp_file
> > +   structures within.  */
> > +void
> > +file_sublist::del (void *p)
> > +{
> > +  file_sublist *e = static_cast<file_sublist *> (p);
> > +  if (e->next)
> > +    {
> > +      file_sublist::del (e->next);
> > +      free (e->next);
> > +    }
> > +}
> > +
> >  /* Initialize everything in this source file.  */
> >  void
> >  _cpp_init_files (cpp_reader *pfile)
> > @@ -1442,6 +1539,10 @@ _cpp_init_files (cpp_reader *pfile)
> >                                         NULL, xcalloc, free);
> >    pfile->dir_hash = htab_create_alloc (127, file_hash_hash,
> > file_hash_eq,
> >                                         NULL, xcalloc, free);
> > +  pfile->pragma_once_files = htab_create_alloc (127,
> > +                                       file_quick_idempotency_attr
> > s::hash,
> > +                                       file_sublist::eq,
> > file_sublist::del,
> > +                                       xcalloc, free);
> >    allocate_file_hash_entries (pfile);
> >    pfile->nonexistent_file_hash = htab_create_alloc (127,
> > htab_hash_string,
> >                                                    
> > nonexistent_file_hash_eq,
> > @@ -1456,6 +1557,7 @@ _cpp_cleanup_files (cpp_reader *pfile)
> >  {
> >    htab_delete (pfile->file_hash);
> >    htab_delete (pfile->dir_hash);
> > +  htab_delete (pfile->pragma_once_files);
> >    htab_delete (pfile->nonexistent_file_hash);
> >    obstack_free (&pfile->nonexistent_file_ob, 0);
> >    free_file_hash_entries (pfile);
> > diff --git a/libcpp/internal.h b/libcpp/internal.h
> > index badfd1b40da..9c3c46df335 100644
> > --- a/libcpp/internal.h
> > +++ b/libcpp/internal.h
> > @@ -485,6 +485,9 @@ struct cpp_reader
> >       been used.  */
> >    bool seen_once_only;
> >  
> > +  /* Optimization for #pragma once.  */
> > +  struct htab *pragma_once_files;
> > +
> >    /* Multiple include optimization.  */
> >    const cpp_hashnode *mi_cmacro;
> >    const cpp_hashnode *mi_ind_cmacro;
> > -- 
> > 2.34.1
> > 
>